Flashbots & MEV-Boost: How Block Builders, Relays & Bundles Power Ethereum MEV

Flashbots fundamentally changed how MEV is extracted on Ethereum. Before Flashbots, searchers competed in chaotic priority gas auctions that congested the network and wasted millions in failed transactions. Today, the Flashbots ecosystem — including MEV-Boost, specialized block builders, and relay infrastructure — processes over 90% of Ethereum blocks. This guide explains every layer of the stack, from Proposer-Builder Separation to the bundle mechanics that power bots like JaredFromSubway.

What is Flashbots and Why Was It Created?

Flashbots is an open research and development organization focused on mitigating the negative externalities of Maximal Extractable Value (MEV) on Ethereum. Founded in early 2021, Flashbots emerged from the realization that MEV extraction was causing serious problems for the Ethereum network: failed transactions clogging blocks, gas price spikes that priced out regular users, and an increasingly opaque relationship between miners and sophisticated traders.

The core idea behind Flashbots was simple but transformative: create a private, off-chain communication channel between MEV searchers and block producers. Instead of competing in the public mempool by bidding up gas prices, searchers could submit sealed bids directly to block builders. This reduced network congestion, lowered gas costs for regular users, and made MEV extraction more efficient for everyone involved.

Flashbots shipped three key products: Flashbots Auction (the original searcher-to-miner channel), MEV-Boost (middleware for Ethereum's proof-of-stake architecture), and Flashbots Protect (an RPC endpoint that shields regular users from front running and sandwich attacks). Together, these tools restructured how value flows through Ethereum's transaction supply chain.

Pre-Flashbots: Priority Gas Auctions and Network Chaos

Before Flashbots, MEV extraction was a brutal free-for-all. Searchers who spotted profitable opportunities — arbitrage, liquidations, sandwich attacks — had only one way to ensure their transactions executed first: pay more gas. This led to Priority Gas Auctions (PGAs), where multiple bots would compete by repeatedly replacing their transactions with higher and higher gas prices.

PGAs were devastating for the network. During peak DeFi activity in 2020 and 2021, gas prices regularly spiked to hundreds of gwei as bots fought over profitable transactions. Regular users trying to make simple transfers or swaps were priced out entirely. Research by Flashbots estimated that PGAs wasted over $1.4 billion in gas fees and failed transactions during 2020 alone.

The failed transactions were particularly wasteful. When multiple bots competed for the same opportunity, only one could win — but every losing bot still paid gas for their reverted transaction. Block space was consumed by failed MEV attempts, reducing throughput for legitimate users. PGAs also created a negative feedback loop: as more bots entered the market, gas wars intensified, which attracted even more attention and capital.

The front running problem extended beyond gas auctions. Miners could privately reorder transactions within blocks they produced, creating an opaque layer of value extraction that was impossible to audit. Some miners ran their own MEV strategies, giving them an unfair advantage that undermined Ethereum's neutrality.

Proposer-Builder Separation (PBS) Explained

Proposer-Builder Separation is the architectural innovation that makes modern MEV extraction possible. PBS splits the traditional block producer role into two specialized functions: builders who construct blocks to maximize value, and proposers (validators) who select and propose the most profitable block to the network.

Before PBS, validators were responsible for both selecting transactions and ordering them within blocks. This meant validators needed sophisticated MEV software to compete, centralizing power among those with the most advanced infrastructure. PBS eliminates this requirement — validators simply choose the block header that pays them the most, without needing to understand MEV strategies at all.

The separation creates a competitive marketplace for block construction. Builders compete to include the most valuable transaction bundles, paying validators increasingly higher bids. This competition ensures that MEV value is distributed efficiently: searchers pay builders, builders pay validators, and the competitive dynamics push most of the value toward the validators who secure the network.

PBS is not yet enshrined in Ethereum's protocol layer — it currently operates through MEV-Boost as out-of-protocol software. Ethereum researchers are working on enshrined PBS (ePBS) as part of the protocol roadmap, which would make builder-proposer separation a native feature of the consensus layer.

How MEV-Boost Works: The Validator-Relay-Builder Pipeline

MEV-Boost is the open-source middleware developed by Flashbots that implements PBS on Ethereum's proof-of-stake network. It sits between validators and the block construction process, enabling validators to receive blocks from a competitive marketplace of builders. Over 90% of Ethereum validators run MEV-Boost, making it the backbone of the MEV supply chain.

The pipeline works in three stages:

Stage 1 — Searchers submit bundles to builders. MEV bots like JaredFromSubway detect profitable opportunities in the mempool and construct transaction bundles. These bundles are sent to one or more block builders via the eth_sendBundle RPC method. Each bundle includes a payment (tip) to the builder, typically sent as a direct transfer to the builder's coinbase address.

Stage 2 — Builders construct blocks and submit to relays. Builders receive bundles from many searchers simultaneously. They assemble the most profitable combination of bundles and regular transactions into a complete block. The constructed block is then submitted to one or more relays along with a bid representing the total payment to the validator.

Stage 3 — Relays deliver blocks to validators. Relays are trusted intermediaries that validate the builder's block and bid, then present the block header to the proposing validator. The validator sees only the bid amount, not the block contents, preventing them from stealing MEV. When the validator selects the highest bid and signs the block header, the relay releases the full block body for propagation.

What Are Bundles? Atomic Transaction Groups and eth_sendBundle

Bundles are the fundamental building block of Flashbots-based MEV extraction. A bundle is an ordered list of transactions that are submitted to a block builder as an atomic group — either all transactions in the bundle execute in the specified order, or none of them execute. This atomicity is critical for MEV strategies that depend on precise transaction ordering.

Consider a sandwich attack: the MEV bot needs to place a buy transaction before the victim's swap and a sell transaction after it. If only the buy executed without the sell, the bot would be left holding tokens at a loss. Bundles guarantee that the buy, the victim's transaction, and the sell all execute together in exact order, or the entire bundle is discarded.

Bundles are submitted via the eth_sendBundle JSON-RPC method, which accepts the following parameters: an array of signed transactions in execution order, a target block number, optional minimum and maximum timestamps, and a list of transaction hashes that must be present in the block for the bundle to be valid. The target block number ensures bundles are only considered for the intended block, preventing stale bundles from executing in later blocks.

Advanced searchers like JaredFromSubway submit bundles to multiple builders simultaneously, maximizing the probability of inclusion. Each bundle includes a tip calculated to be competitive enough for inclusion while preserving profit margin. The tip is typically the last transaction in the bundle — a direct ETH transfer to the builder's coinbase address.

Builder Tips and Coinbase Transfers

The economics of MEV-Boost revolve around builder tips. When a searcher submits a bundle, they include a payment to the block builder as an incentive for inclusion. This payment is almost always a direct transfer to the builder's block.coinbase address — the address that receives the block reward and priority fees.

Coinbase transfers are preferred over high gas prices because they offer several advantages. First, they are only paid if the bundle executes successfully — a reverted bundle costs the searcher nothing beyond the base fee. Second, they allow precise payment amounts rather than the imprecise cost of inflated gas limits. Third, they keep the effective gas price of bundle transactions low, making bundles more attractive for builders to include.

The competitive dynamics of the builder market mean that tip rates have converged around 90-99% of extracted MEV. Top searchers like JaredFromSubway operate at a 99.9% bribe rate, meaning nearly all extracted value is passed to builders and validators. The searcher keeps only a thin margin on each transaction, but the massive volume — thousands of bundles per day — makes this highly profitable in aggregate.

Key Block Builders: Beaverbuild, Titan, and BuilderNet

The block builder ecosystem is dominated by a handful of highly specialized operators. Understanding who builds Ethereum's blocks is essential for any serious MEV operation, because the choice of builder directly affects bundle inclusion rates and latency.

Beaverbuild is consistently one of the top block builders by market share, often constructing 30-40% of all Ethereum blocks. Beaverbuild is known for aggressive optimization, low latency block submission, and high inclusion rates for competitive bundles. Their infrastructure processes thousands of bundles per block slot.

Titan Builder is another dominant force, frequently trading the top position with Beaverbuild. Titan is recognized for reliability and consistently competitive bids. Many professional MEV operations, including JaredFromSubway, submit bundles to both Beaverbuild and Titan simultaneously to maximize inclusion probability.

BuilderNet, launched by Flashbots in November 2024, represents a fundamentally different approach. Rather than a single centralized operator, BuilderNet is a distributed, permissionless block building network. Multiple parties contribute to block construction using trusted execution environments (TEEs), reducing the trust assumptions required of any single builder. BuilderNet aims to decentralize the builder role and prevent the builder market from becoming a duopoly.

Other notable builders include rsync-builder, Blocknative, and various proprietary builders run by large trading firms. The builder market is intensely competitive — builders that fail to offer competitive bids or reliable inclusion quickly lose market share.

How JaredFromSubway Uses Flashbots

JaredFromSubway is one of the most prolific users of Flashbots infrastructure on Ethereum. On-chain data reveals the scale: JaredFromSubway paid 851 ETH to block builders in a single two-week period, operating at a 99.9% bribe rate. This means for every 1,000 ETH of MEV extracted, approximately 999 ETH goes to builders and validators, with JaredFromSubway retaining roughly 1 ETH in net profit per 1,000 ETH of volume.

This razor-thin margin works because of volume. JaredFromSubway processes thousands of sandwich attacks and arbitrage opportunities daily, submitting bundles to multiple top builders including Beaverbuild and Titan. The bot's custom smart contracts are optimized for minimal gas consumption, and its mempool monitoring infrastructure detects profitable transactions within milliseconds.

JaredFromSubway's Flashbots strategy involves several key optimizations. First, the bot simulates every potential bundle locally using a forked EVM before submission, ensuring only profitable bundles are sent. Second, bundles are submitted to all major builders simultaneously with precisely calculated tips. Third, the bot uses adaptive tip pricing — during high-competition periods, tips increase automatically to maintain inclusion rates.

To learn more about building similar MEV infrastructure, see our guide on how to build an MEV bot.

Flashbots Protect for Users vs. Flashbots for Searchers

Flashbots serves two very different audiences with distinct products. For regular Ethereum users, Flashbots Protect is a free RPC endpoint that routes transactions through a private channel, preventing them from being seen in the public mempool. This shields users from sandwich attacks and other forms of front running.

When a user sends a transaction through Flashbots Protect, it bypasses the public mempool entirely and is sent directly to block builders. Since MEV bots like JaredFromSubway monitor the public mempool for targets, transactions routed through Protect are invisible to these bots. The trade-off is slightly longer confirmation times, since the transaction is only visible to builders rather than the entire network.

For searchers and MEV bots, Flashbots provides the bundle submission infrastructure described above. The Flashbots RPC accepts bundles via eth_sendBundle and routes them to connected builders. Searchers also access Flashbots' MEV-Share protocol, which allows users who opt in to share their order flow with searchers in exchange for a portion of the MEV extracted. This creates a more equitable distribution of MEV value.

The tension between these two products reflects the broader MEV ecosystem: Flashbots simultaneously helps users avoid MEV extraction and helps bots perform it more efficiently. JaredFromSubway and other top bots have adapted to MEV-Share by developing strategies that work within its constraints, sharing a percentage of extracted value back with the originating user.

The Future: SUAVE and Encrypted Mempools

Flashbots is building toward a future where MEV extraction is more decentralized, transparent, and equitable. The most ambitious project is SUAVE (Single Unified Auction for Value Expression) — a separate blockchain designed to serve as a decentralized transaction ordering platform for all EVM chains.

SUAVE aims to replace the current trusted relay model with a permissionless, cryptographically secured system. Instead of relying on centralized relays to mediate between builders and proposers, SUAVE uses trusted execution environments and cryptographic commitments to ensure fair ordering without trust assumptions. Searchers would submit encrypted bids to SUAVE, which would determine optimal transaction ordering across multiple chains simultaneously.

Encrypted mempools represent another frontier. Several research teams are exploring systems where transactions are encrypted until they are committed to a block, making pre-execution MEV extraction impossible. Technologies like threshold encryption, commit-reveal schemes, and delayed decryption could fundamentally change how JaredFromSubway and other MEV bots operate.

However, even in an encrypted mempool world, some forms of MEV persist. Back-running arbitrage (correcting price discrepancies after large trades) would still be possible because the opportunity only appears after transaction execution. Statistical MEV strategies that don't require seeing specific transactions would also survive. The MEV landscape will evolve, but it won't disappear — and JaredFromSubway's infrastructure is designed to adapt to whatever comes next.

Frequently Asked Questions

What is Flashbots and how does it work?

Flashbots is an open research and development organization that builds infrastructure for transparent MEV extraction on Ethereum. It provides a private communication channel between searchers and block builders, allowing MEV bots like JaredFromSubway to submit transaction bundles without exposing them to the public mempool. This eliminates failed transactions, reduces gas waste, and creates a more efficient marketplace for block space.

What is MEV-Boost?

MEV-Boost is open-source middleware that implements Proposer-Builder Separation (PBS) on Ethereum. It allows validators to outsource block construction to specialized block builders who compete to create the most profitable blocks. Relays act as trusted intermediaries, ensuring builders can't cheat validators and validators can't steal MEV. Over 90% of Ethereum blocks are produced through MEV-Boost.

What is a Flashbots bundle?

A Flashbots bundle is an atomic group of transactions submitted together via the eth_sendBundle RPC method. Bundles guarantee that all transactions execute in the exact order specified, or none execute at all. JaredFromSubway uses bundles to execute sandwich attacks with precise ordering: a front-run buy, the victim's swap, and a back-run sell — all guaranteed to execute atomically.

What is Proposer-Builder Separation (PBS)?

Proposer-Builder Separation splits the role of block production into two specialized parties: builders who construct blocks to maximize value, and proposers (validators) who select and propose the most profitable block. This specialization allows MEV extraction to happen efficiently without validators needing to run complex strategies themselves. PBS is currently implemented through MEV-Boost and is being considered for enshrinement in Ethereum's protocol.